The present invention relates to a high-damping probe suitable for use in measurement of the thickness, internal void, compressive strength, etc. of concerete structures by the ultrasonic measuring method.
The present invention also relates to an ultrasonic measuring apparatus for use in quality control and deterioration testing of concrete structures or in non-destructive testing of timbers, fiber reinforced plastics (FRP) and so forth. More particularly, the present invention pertains to an ultrasonic measuring apparatus using a high-damping probe which is capable of accurately measuring the sound velocity of the shear wave and also relates to an apparatus using a high-damping probe which is capable of accurately measuring the thickness of and internal voids in an object of testing.
In the fields of building and civil engineering, various kinds of measurement, such as measurement of the thickness, internal void, compressive strength, etc. of concrete or other materials have heretofore been conducted for the purpose of structural examination, execution control or quality control of concrete structures or civil engineering constructions. These measuring processes are generally carried out in a non-destructive manner using ultrasonic waves. Conventional ultrasonic probes (ultrasonic sensors) and ultrasonic measuring apparatus suffer, however, from the following problems.
Convenional ultrasonic probes will first be explained. A typical conventional ultrasonic probe comprises a transducer 72 which is secured to a front panel 71 which is in turn secured to an opening provided in a metallic casing 70, as shown in FIG. 6. In operation, a plug 73 which is attached to the casing 70 is supplied with an electric pulse from an ultrasonic testing device (not shown), and this electric pulse is transmitted by the transducer 72 after being converted into an ultrasonic pulse. The conventional probe shown in FIG. 6, however, involves the problem that, when an electric pulse is applied thereto, the transmission wave oscillates for a long period of time, as shown in FIG. 7, and consequently the reflected wave and the transmission wave are superposed one upon the other, thus making it exceedingly difficult to clearly discriminate the reflected wave from the transmission wave.
The conventional ultrasonic measuring method and apparatus involve the following problems.
Measurement of the compressive strength of concrete has heretofore been conducted by employing an arrangement such as that shown in FIG. 8(a).
Referring to FIG. 8(a), a transmitting probe 74 that performs transmission of an ultrasonic pulse is disposed on one face of concrete 75 which is an object of measurement of compressive strength, and a receiving probe 76 that detects the wave transmitted through the concrete 75 is disposed on the other face of the concrete 75. The ultrasonic pulse transmitted from the transmitting probe 74 is partly transmitted through the concrete 75 and partly reflected inside it. The transmitted wave is detected by the receiving probe 76, and the waveform of the received signal is displayed on an oscilloscope (not shown). Ultrasonic information that is obtained directly from the waveform of the received signal is a travelling period of the transmitted wave. From the travelling period and the thickness of the concrete 75 obtained in advance by another method, the sound velocity of the ultrasonic pulse is obtained, and the compressive strength of the concrete 75 is determined on the basis of the obtained sound velocity and a calibration curve prepared in advance.
Ultrasonic waves used in ultrasonic testing involve longitudinal and shear waves, and information representative of the sound velocity of the shear wave is particularly essential for obtaining the elastic modulus, which is an important factor of the compressive strength. Accordingly, a shear wave probe is employed to measure the compressive strength. In the shear wave probe 77, however, not only a shear vibration component in the diametrical direction of the transducer but also a thickness vibration component in the direction of the thickness of the transducer are generated, as shown in FIG. 8(b); therefore, not only a shear wave component is generated due to the shear deformation, but also a longitudinal wave component is simultaneously generated due to the thickness deformation. On the oscilloscope is therefore displayed a waveform in which the reflected shear wave component 78 and the reflected longitudinal wave component 79 interfere with each other, as exemplarily shown in FIG. 8(c). Accordingly, it is exceedingly difficult to read only the travelling period of the shear wave. It should be noted that the abscissa axis in FIG. 8(c) represents time. Thus, the conventional shear wave probe suffers from the problems that it is difficult to measure the sound velocity with high accuracy and also difficult to analyze the shear wave only.
In the conventional process for measuring compressive strength, the single-pulse excitation system is often employed. However, since it is difficult with the single-pulse excitation system to generate an ultrasonic wave which efficiently travels through concrete, ultrasonic data obtained by reception of the transmitted ultrasonic wave is likely to be unreliable. In addition, since the conventional probes are of the free vibration type, it is only possible to measure the sound velocity from the received wave and it is difficult to measure other useful ultrasonic data such as the frequency of the received wave and the amplitude of the received pulse. Since the compressive strength of concrete has heretofore been estimated from the propagation velocity as described above, the prior art involves large errors, i.e., .+-.100kg/cm.sup.2, which cannot possibly be tolerated.
As a method of measuring the compressive strength of concrete, a method which uses in combination the sound velocity method and Schmidt rebound hammer is known. This method, however, involves a complicated technique, and no effective equation to estimate the compressive strength of concrete has yet been established.
The following is a description of the conventional process for measuring the thickness of and an internal void in a concrete slab and problems experienced therewith.
Measurement of the thickness of and an internal void in a concrete slab has heretofore been conducted by single or double probe technique. The single probe technique is an inspection method wherein an ultrasonic pulse is transmitted from a probe 80 to a concrete slab 81 as being a testing object and the reflected wave from the inside of the concrete slab 81 is received by the same probe 80, as shown in FIG. 9, the waveform of the received signal being displayed on a display means (not shown), for example, an oscilloscope. Thus, the travelling period of the ultrasoic pulse is obtained from the waveform displayed, and the position of an internal void 82 or the thickness of the concrete slab 81 is determined on the basis of the relationship between the obtained travelling period and a reference sound velocity of the ultrasonic wave in the concrete slab 81 obtained by another method. In measurement of the thickness of the concrete slab 81 or the position of the internal void 82 by the described single probe technique, however, it is difficult to clearly discriminate the reflected wave because the conventional probe involves the phenomenon that the transmission wave oscillates for a long period of time, as shown in FIG. 7, and hence the transmitted pulse is undesirably superposed upon the reflected wave from the bottom of the concrete slab 81 or the internal void 82.
FIG. 10 exemplarily shows an arrangement employed to measure the thickness of concrete or an internal void therein by the method generally known as double probe technique. A transmitting probe 83 and a receiving probe 84 are disposed on the same face. An ultrasonic pulse is transmitted from the probe 83, and the reflected wave from the bottom of the concrete 85 or the internal void 86 is detected by the receiving probe 84. The waveform of the reflected wave is displayed on a display means (not shown), for example, an oscilloscope, and the travelling period of the ultrasonic pulse is read from the displayed waveform, thereby measuring the thickness of the concrete 85 or the position of the internal void 86. In the conventional ultrasonic measuring apparatus and probes, however, the frequency component of the ultrasonic wave used is unconditionally determined by the performance of each individual ultrasonic measuring apparatus. There are therefore cases where measurement cannot be effected because of scattering of attenuation of the ultrasonic pulse, depending upon the cement and aggregate content and type. Moreover, since the transmitted pulse oscillates for a long period of time as shown in FIG. 7, measurement cannot be effected with high accuracy.
It is necessary in order to measure the thickness of an object of inspection to obtain a distinct received wave. With the conventional probe, however, the propagation characteristics of the ultrasonic wave in the object are not always good and it is therefore difficult to obtain a distinct received wave. Accordingly, measurement is difficult to conduct and it is impossible to effect measurement with high accuracy.
To measure the thickness, a reference sound velocity in each individual object of inspection is needed and it is common to measure a reference sound velocity in an object of inspection in advance of measurement of the thickness thereof. However, there has heretofore been a problem that it is difficult to properly dispose two probes for transmission and reception for measurement of the reference sound velocity. More specifically, measurement of the reference sound velocity is generally effected by the through transmission method using two normal probes, and it is therefore necessary to dispose normal probes for transmission and reception on the obverse and reverse faces, respectively, of an object of inspection. Accordingly, if it is impossible to ensure a space which is sufficiently large to place the receiving probe and an operator who operates it, measurement itself cannot be carried out.
In addition, an object of inspection must have an opening in order to enable measurement of the reference sound velocity by the through transmission method and measurement of the thickness by the echo, or reflection, method. However, many of objects which need measurement of the thickness have no openings; therefore, in many cases it is impossible to measure the reference sound velocity.
Thus, the conventional probes, ultrasonic measuring methods and apparatuses involve many problems.